Inborn errors of amino acid metabolism are individually rare but collectively represent significant causes of mortality and morbidity in the Pediatric population. The most comprehensive information source for these disorders list over 100 known disorders of amino acid metabolism. Hoever, the rarity of each individual disorder has hempered our understanding of these diseases. Indeed, many of the disorders are defined by fewer than 100 published cases. Their very rarity means that each patient should be evaluated in such a way that we not only meet the medical needs of the patient, but also further delineate and understand the natural history of these diseases.